Retirement and Deferred Compensation Plans - Schedule of Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Loss (Income) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Periodic Benefit Cost
Settlements	$ 10.2
Pension Plan
Net Periodic Benefit Cost
Service cost		$ 21.0	$ 15.1	$ 10.9
Interest cost		9.4	12.7	11.6
Expected return on assets		(13.5)	(12.9)	(10.6)
Settlements		13.8	0.4
Net loss		2.7	1.2	1.4
Prior service cost		0.7	0.7	0.6
Net periodic benefit cost		34.1	17.2	13.9
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income/Loss
Net loss (gain)		44.1	45.5	(16.6)
Prior service cost		0.4	1.0	1.4
Amortization of net loss		(6.3)	(1.6)	(1.4)
Amortization of prior service cost		(0.7)	(0.7)	(0.6)
Total recognized in other comprehensive income/loss		37.5	44.2	(17.2)
Total recognized in net periodic benefit cost and other comprehensive income/loss		71.6	61.4	(3.3)
Retiree health care plan
Net Periodic Benefit Cost
Interest cost		0.4	0.5	0.5
Net loss		0.1		0.1
Prior service cost		(0.8)	(0.8)	(0.8)
Net periodic benefit cost		(0.3)	(0.3)	(0.2)
Other Changes in Plan Assets and Benefit Obligation Recognized in Other Comprehensive Income/Loss
Net loss (gain)		1.1	0.8	(0.5)
Amortization of net loss		(0.1)		(0.1)
Amortization of prior service cost		0.8	0.8	0.8
Total recognized in other comprehensive income/loss		1.8	1.6	$ 0.2
Total recognized in net periodic benefit cost and other comprehensive income/loss		$ 1.5	$ 1.3